Add updater rewriterules to make 13.5a10 a watershed

It won't be an actual watershed at this point, since we've already released several 14.0aX, but it'll allow us to test that everything works when we'll have to do it for the actual release.

Also, we'll have to do an actual 14.0aX watershed to reduce the cognitive load of the differences between alphas and stables.

Notice that when we extend this to 14.0aX, we might want to include Mullvad Browser.